Plastisols are suspensions of polymer particles in plasticizer that flow as a liquid for application and are then heated to form flexible plasticized product having a fixed shape. Plastisols are useful as a coating, binder, adhesive, sealant or protective layer in a wide variety of applications, including protective coatings for metal finishes, UV films for outdoor products, and automotive products (e.g. underbody coatings and interior coverings).
Plastisol compositions are also used as a textile ink for screen-printing applications forming graphics and other images on the textiles. Most common among these imaged textiles are T-shirts with images of famous entertainers, college names, witty sayings, etc.
The ink composition is a plastisol, because the means of application of the colorful liquid ink utilizes the fluid properties of the plastisol before heating and/or pressure causes the base resin in the plastisol to cure into a colorful solid on the surface of the textile.
Historically, most plastisols were a combination of polyvinyl chloride (PVC) resin particles dispersed in and swelled by phthalate-based plasticizers. However, due to environmental and health safety concerns, the composition of these inks is under increasing scrutiny. In particular, the Consumer Product Safety Improvement Act of 2008 and the European Community regulation on chemicals, Registration, Evaluation, Authorisation and Restriction of Chemical substances (EC 1907/2006), restrict the following phthalates in certain articles (hereinafter referred to as “restricted phthalates”): bis(2-ethylhexyl) phthalate (DEHP, DOP), dibutyl phthalate (DBP), benzyl butyl phthalate (BBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), and di(n-octyl) phthalate (DNOP).
The use of acrylic resins in place of PVC, which acrylics also do not use restricted phthalates, is especially attractive and has been extensively researched. Nonetheless, acrylic resin plastisol inks continue to have shorter storage stability, less desirable hand-feel characteristics of softness without tackiness, less toughness/durability and can be more difficult to process using the processes and techniques otherwise used for PVC plastisol, which continues to be prevalent in the industry.
To address some of the problems associated with acrylic-based plastisol resins, inventors of a number of patents and patent applications have proposed the use of acrylic core-shell polymers, namely: U.S. Pat. No. 4,199,486 (Boessler et al.); U.S. Pat. No. 5,324,762 (Overend et al.); U.S. Pat. No. 6,355,712 (Schultes et al.); U.S. Pat. No. 6,433,048 (Kasai); and US 2010/0069566 (Mae). These patents and published patent applications teach that a core-shell structure is effective in improving the storage stability of acrylic plastisol. Unfortunately, improvements in storage stability of the core-shell acrylic resins are usually accompanied by a decrease in certain physical properties significant to the use of acrylic resins as a plastisol resin, such as the toughness of the cured resin, which also affects its durability over time.
To address the poor strength, elongation and durability issues of plastisols made with acrylic polymer resins, several persons have described using interpenetrating polymer networks (IPNs) where a blocked isocyanate-functionalized urethane prepolymer is added to the acrylic plastisol to form a separate polyurethane or polyurea phase when the plastisol composition is cured. Examples of this approach are U.S. Pat. No. 6,916,869 Eto et al.; US 2006/0173110 (Baba); and U.S. Pat. No. 7,332,539 (Nakayama et al.), WO 2013/016265 (Suzuki), US 2005/282990 (Schoenfeld at al.), U.S. Pat. No. 6,809,147 (Ohno et al.), and U.S. Pat. No. 6,559,193 (Nonoyama et al.). These examples teach the crosslinking of only the polyurethane resin in the cured plastisol.
The addition of a polyurethane resin to the acrylic plastisol is recommended in the above patents and published applications, because polyurethanes generally have better toughness compared to acrylic polymers. However, in a plastisol having a two-resin system the acrylic polymer and polyurethane are predisposed to form separate domains due to their incompatibility, diminishing the toughness and durability of the plastisol.
Therefore, the industry still lacks a solution to the deficiencies of the various prior approaches to a non-PVC plastisol ink, which does not rely upon a restricted phthalate plasticizer and, which, in particular, meets all the requirements for screen-printing of textiles.